Variable subscriptions not returning statuscode change on disconnect?

[jmarshall9120]

According the OPC UA spec:

The Value Attribute is used when monitoring Variables. Variable values are monitored for a change in value or a change in their status.
(https://reference.opcfoundation.org/v104/Core/docs/Part4/5.12.1/)

That's the from the spec for monitored items. Also according to the spec:

The preferred mechanism for a Client to monitor the connection status is through the keep-alive of the Subscription. A Client should subscribe for the State Variable in the ServerStatus to detect shutdown or other failure states. If no Subscription is created or the Server does not support Subscriptions, the connection can be monitored by periodically reading the State Variable.
(https://reference.opcfoundation.org/v104/Core/docs/Part4/6.7/)

When I look at an implementation like uaexpert and do as the spec directs I get this:

It appears to detect the disconnection through a change event that was picked up for the StatusCode of node Server_ServerStatus_State.

When I do a simple implementation with the opcua-asyncio library I get this result:

# %%
import config
from asyncua import Node, ua, Client
import asyncio
import logging

_logger = logging.getLogger('Test')

class SubscriptionHandler:
    """
    The SubscriptionHandler is used to handle the data that is received for the subscription.
    """
    def datachange_notification(self, node: Node, val, data):
        """
        Callback for asyncua Subscription.
        This method will be called when the Client received a data change message from the Server.
        """
        _logger.warning('datachange_notification %r %s %s', node, val, data)


client = Client(config.OPCUA_URL_CODESYS)
client.set_user(config.OPCUA_USER_CODESYS)
client.set_password(config.OPCUA_PASSWORD_CODESYS)
await client.connect()

status_node_id = Node(client, ua.ObjectIds.Server_ServerStatus_State)
handler = SubscriptionHandler()
subscription = await client.create_subscription(500, handler)
await subscription.subscribe_data_change(
    nodes=[status_node_id],
    attr=ua.AttributeIds.Value, 
    queuesize=50, 
    monitoring=ua.MonitoringMode.Reporting,
)
_logger.warning('listening...')
await asyncio.sleep(60)

await subscription.delete()
await client.disconnect

Revised values returned differ from subscription values: CreateSubscriptionResult(SubscriptionId=2, RevisedPublishingInterval=500.0, RevisedLifetimeCount=16200, RevisedMaxKeepAliveCount=5400)
listening...
datachange_notification Node(NodeId(Identifier=2259, NamespaceIndex=0, NodeIdType=<NodeIdType.FourByte: 1>)) 0 DataChangeNotification(<asyncua.common.subscription.SubscriptionItemData object at 0x00000205BD7554C0>, MonitoredItemNotification(ClientHandle=201, Value=DataValue(Value=Variant(Value=0, VariantType=<VariantType.Int32: 6>, Dimensions=None, is_array=False), StatusCode_=StatusCode(value=0), SourceTimestamp=datetime.datetime(2022, 6, 17, 19, 1, 30, 721000), ServerTimestamp=datetime.datetime(2022, 6, 17, 19, 1, 30, 721000), SourcePicoseconds=None, ServerPicoseconds=None)))

In this case I stopped the sever shortly after the client began listening but no disconnection was detected.

Obviously this easy to solve by polling the server, but according to the OPCUA spec, I shouldn't be doing that. Any idea why the subscription doesn't report changes to StatusCode?